Call routing in a mobile telecommunication network is different from and generally more complicated than that in a fixed telecommunication network. In a fixed network such as a Public Switched Telephone Network (PSTN), a user terminal is permanently connected to a central office, or a central switch. In a mobile telecommunication network on the other hand, a subscriber can roam freely from the service area of one switch to that of another. That is to say, the switch which serves the mobile subscriber—providing switching functions including call routing—may change from time to time.
From the perspective of a calling party who wishes to place a call to a mobile recipient subscriber, a change in the serving switch of the recipient subscriber is invisible. Whether the recipient subscriber is located within the service area of his “home” switch, i.e. the switch at which he registers with the mobile communication network, or is under the service of a different switch, the calling party always dials one and the same telephone number, or directory number, of the recipient subscriber. This number is usually called the Mobile Subscriber ISDN (MSISDN) of the recipient subscriber.
In order to route the call to the recipient subscriber, the switching system of the mobile network internally assigns a number that is different from the MSISDN, a so-called roaming number, to the call and uses this roaming number to determine how to route the call to the terminating switch of the call, that is, to the particular switch which serves the recipient subscriber at the time of the call. The roaming number is often called the Mobile Station Roaming Number (MSRN). MSRNs are related to the geographical numbering plan, and they are not assigned to nor visible to any subscriber. In summary, by means of the MSRN number associated with a mobile terminating call, the switching system of the mobile communication network can route the call to the terminating switch serving the recipient subscriber of the call. A similar mechanism is used for handovers.
With the development of distributed computing, more and more switches in mobile telecommunication networks are being implemented as distributed systems, also called switch clusters. In the language of distributed computing, a “cluster” is a set of loosely coupled computing devices that work together closely so that in many aspects they can be viewed as though they are a single computing device. A cluster usually contains a number of cluster members, or blades. A cluster member is a computing device having all the essential functional components to be considered as a computer while lacking certain features of a stand-alone device for the consideration of space, power, etc.
Applying the cluster paradigm, each switch can be configured as a switch cluster having a plurality of cluster members. That is, a service area which used to be served by one stand-alone switch can now be served by a highly compact switch cluster. The cluster approach provides the benefits of load balancing and high availability, to name a few. For instance, in order to keep the load for each cluster member as even as possible, a distribution mechanism distributes all subscribers served by the service area of the switch cluster over all cluster members.
In principle, each cluster member can perform the switching functionalities just as a conventional stand-alone switch does. For example, each cluster member can handle mobile originating traffic, can receive an incoming call, can route the incoming call to the switch cluster who currently serves the recipient subscriber of the call, and can terminate the incoming call if the recipient subscriber is served by the cluster member itself. Thus, in a distributed switching system with one or more switch clusters wherein each cluster owns a plurality of cluster members, it is necessary for the switching system to determine, for each incoming mobile terminating call, not only the terminating switch cluster but also the very cluster member (in the terminating switch cluster) who serves the recipient subscriber of the call.
Accordingly, there is a need for a technique to efficiently handle mobile terminating calls, and handover calls, in a distributed switching system.